1. Field of the Invention
This invention relates generally to forward error correction (FEC) encoding and decoding in optical transmission systems and more particularly to FEC coding to provide for “averaging” of the bit error rate (BER) across multiple signal channels.
2. Description of the Related Art
There is an ever increasing demand on wavelength division multiplexing systems for increase in capacity over the existing optical telecommunication fiber transport systems by increasing the rate and distance at which data signals are transmitted. It is not an easy approach to improve system performance by merely adding more signal channels, decrease the channel wavelength spacing, increase the channel power or increase the data signal rate of transmission, as this usually enhances further system losses due to the nonlinearity effects of the fiber transport medium, such as chromatic dispersion (CD), polarization mode dispersion (PMD) and signal crosstalk. Many techniques have been devised to increase system performance in order to increase the signal reach of the network thereby rendering an optical transmission network more cost effective. For example, in order to increase the signal data rate or modulated frequency toward increasing system capacity, the signal power must be correspondingly increase to achieve the same channel performance as was attained at lower signal data rates, which performance is measured as a bit error rate (BER). The deployment of optical amplifiers has been also an approach to increase the signal reach in an optical span by providing signal gain in the optical domain. This has lead to a procession of research and development to provide all-optical networks where the client signals are handled in the optical domain through out the network span, such as, all optical ADMs, crossconnects and 3R signal processing. Whether such all-optical systems will deliver higher performance at lower effective costs remains to be seen.
One manner of increasing system performance in an effective less-cost manner is the deployment of forward error correction (FEC) as known in the art. The use of FEC encoding and decoding improves system performance in a cost effective way by providing overall enhancement to the channel signals in the electrical domain by providing what is termed, coding gain, to the channels signals. Coding gain is an indication of the enhancement to signal-to-noise ratio (SNR) achieved through the use of FEC encoding. Gain coding helps to minimize phase noise or jitter as well as improve upon the effects brought about by optical nonlinearities by providing a way to provide additional information to the client payload signal that may be utilized at the receiver to reduce signal bit error rate (BER).
Thus, FEC is increasing deployed for accurately transmitting client payload in optical transmission networks. As shown in FIG. 1, FEC systems add separate redundant data in the transport signals through FEC encoding with N FEC encoders at the transmitter side from a plurality of N data sources. The FEC encoded signals are then transported over multiple network mediums which, because of its nonlinearity characteristics, provide a noisy channel for each of these signals. The FEC encoded information is decoded on the receiver side via FEC decoders, providing signal redundancy, and a means by which error correction in the electrical domain can be used to provide a higher accurate signal recovery such as channel error rates around, or smaller than, 10−12. The FEC corrected signals are provided to N data sinks which may be various types of client payload equipment for receiving the signals through tributary interfaces between the network and such equipment.
During the encoding operation, redundant signal information is added to the signal payload, usually at the end of the end of the signal payload, for example, in the case of the OTN network node interface defined in the ITU-T G.709 where it is added at the end of the optical transport unit (OTU) framing structure. The redundant information allows determination by the receiver FEC system as to whether the received data signals have been corrupted during transit through the network. If corruption has occurred, the incorrect data is detected or identified and corrected employing the coded redundant information. In particular, the FEC decoder decodes the FEC data, generates a signature of the error (syndrome) which in turn is employed to generate an error location and error value polynomial and employs the polynomial to determine the correction to be applied to the data signal.
FIG. 2 illustrates a more recent approach in the use of FEC systems. Such a system is shown in U.S. Pat. No. 6,433,904. In FIG. 2, in the optical transmitter, the data signals from the respective N signal sources are FEC encoded by corresponding N FEC encoders. The FEC encoded signals are EO converted via a corresponding series of optical transmitters, λ1, λ2 . . . λN, after which the generated optical signals are multiplexed by N:1 MUX and the multiplexed signal is launched onto a network fiber medium which are N noisy channels due to the impurities of the medium. The dotted lines in FIG. 2 as well as in other figures are indicative of the respective N noisy channels along the medium. At the receiving end at the optical receiver, the optical multiplexed signal is demultiplexed by a 1:N DEMUX and the individual N optical signals are converted from optical signals to electrical signals, after which they are FEC decoded by N FEC decoders and provided to data sinks via tributary interfaces which may be, for example, client signal equipment.
Also, as illustrated in FIG. 3 herein, which is similar, in part, to FIG. 4 of U.S. Pat. No. 6,433,904, the data source may be a high bit or baud rate source which is FEC encoded and inverse multiplexed where the data signal is demultiplexed by 1:N DEMUX into multiple lower baud rate, N channel signals for signal conversion from electrical to optical signals via optical transmitters, λ1, λ2 . . . λN, and thence the optical generated signals are optically multiplexed for transmission over the optical span. At the optical receiver, the reverse of the forgoing occurs, i.e., the multiplexed optical signal is demultiplexed by a 1:N DEMUX and the optical signals are converted into electrical signals by the optical receivers, λ1, λ2 . . . λN, FEC decoded and then inverse multiplexed to provide, again, a higher baud rate signal which is provided to a data sink, such as client equipment via a client tributary interface. The purpose of FEC systems in FIGS. 3 and 4 of patent '904 is to provide for upgrade in the signal baud rate, such as from a 2.5 Gbps system to meet the requirements of a 10 Gbps system, toward reducing the effects of optical channel impairments at higher data rates in order to achieve an improved system optical signal-to-noise ratio (OSNR). As background, inverse multiplexer and demultiplexer techniques are disclosed in U.S. Pat. No. 5,065,396.
U.S. Pat. No. 6,433,904 also discloses in FIG. 7 therein, in another embodiment, a joint FEC encoder where separate signals from multiple data source are jointly FEC encoded by one FEC encoder and the jointly FEC encoded signals are separately EO converted for transmission on an optical medium. The reverse procedure is accomplished at the optical receiver.
Published U.S. patent application 2002/0114358, published on Aug. 22, 2002, discloses the FEC encoding of multiple data signals which are thereafter combined to form a single FEC encoded electrical signal which is then EO converted via an optical transmitter for transmission on an optical network medium. The reverse procedure is accomplished at the optical receiver. In order to enhance system SNR, the combiner comprises an aharmonic interleaver which interleaves the signal segments or bytes in a manner that they are aharmonic with respect to the transmission data rate. The interleaving has the effect of distributing bits or bytes with each of the separate data streams within a higher data rate signal thereby distributing the effects of noise among the separate signals which thereby effectively reducing the effective SNR across all the separate signal channels.
In the most of the foregoing FEC coding systems, system improvements to be achieved through appropriate FEC encoding is based on coding gain required for the worst case signal channel. This means that the best or better operating channels that have lower levels of jitter and noise are utilizing coding gain that is in excess of what is required or necessary. It is an object of this invention to provide a method and apparatus for the “averaging” of coding gain across all of the channels so that a portion of the coding gain extended to better channels with less noise is shared with channels suffering with more noise.